1. Field of Invention
The present invention relates generally to an improved tip (i.e., spill) seal design for use in axial flow elastic fluid turbines, and more particularly to an improved spillstrip for use therein to permit improved removal of hard metallic particles from the steam flow through such turbines, for the purpose of minimizing blade wear, turbine maintenance and repair, and thus cost of machine operation.
2. Brief Description Of The Prior Art
The use of axially flow elastic fluid turbines, such as axially flow steam turbines, plays a very important role in the production of electric power in our society. In order to produce electrical power from an electrical power generator installed at a power plant, it is necessary to rotate the rotor shaft thereof in a magnetic field produced by the stator field windings of the power generator. Typically, the torque required to rotate the rotor shaft at a sufficient angular velocity is provided by a steam turbine whose output shaft is mechanically coupled to the rotor shaft of the generator. Often, in a typical power plant, there will be a number of steam turbines each driving one or more electrical power generators.
In general, each steam turbine comprises a shaft rotatably supported by bearings which are encased in a housing or casing. In order to rotate the turbine shaft using the momentum of super-heated vapor (i.e., "steam"), a series of turbine stages are sequentially arranged along the axis of the shaft. A boiler, typically located external to the turbine casing, is provided for the purpose of generating steam. External to the turbine casing are steam pipes which are used to conduct the steam from the boiler to particular sections of the turbine, that are typically classified by operating pressure. Along each section of the turbine, there are typically a number of turbine stages.
At each turbine stage, a turbine rotor is fixedly mounted to the turbine shaft. Each turbine rotor has a plurality of blades which radially extend a predetermined distance from the shaft, towards a circumferentially extending shroud band (i.e., cover) that is secured to the tenon portions of the blades. In general, each turbine blade is oriented at an acute angle with respect to the axis of rotation of its rotor. In order that each turbine rotor is permitted to freely rotate with the turbine shaft, the turbine casing has circumferential recesses to accommodate the rotor structures along the shaft. A stationary diaphragm is installed behind each rotor in a circumferential joint formed in the turbine casing. Each turbine has a ring of steam nozzles circumferentially extending about the inner structure of the diaphragm. These nozzles are located at the same radial position as the blades in its associated rotor. The function of each nozzle is to receive steam from the passageways in the turbine casing and to physically direct the steam against the rotating blades of its associated rotor. To establish a "tip seal" with the shroud band of each turbine rotor, a ring of spillstrips is supported from the diaphragm at each stage.
As the steam travels along a helical path through the turbine, a portion of its linear momentum is transformed into the angular momentum of the rotor blades at each turbine stage, thereby imparting torque to the turbine shaft. At each subsequent stage, the pressure of the steam path is typically reduced. Thus at these downstream stages it is often necessary to increase the length of the rotor blades and the size of the associated diaphragms in order to extract kinetic energy from axially flowing steam of reduced pressure.
In recent times, steam turbine design has been concerned primarily with two problem areas, namely: (i) the quality of steam seals between the various stationary and rotating components along the steam flow path in the turbine; and (ii) the wear of components caused by the presence of hard particulate matter (e.g., oxide and other metallic particles) in the steam path through the turbine.
In general, the first problem has been addressed by improved designs in packing rings, retractable packing seals, and seal rings.
Recently, the second problem has been addressed in U.S. Pat. No. 5,271,712 to Brandon, which is incorporated herein by reference in its entirety. In this prior U.S. Patent, Brandon discloses an improved spillstrip having a "through opening" formed in its tip seal portion. While the spillstrip design of U.S. Pat. No. 5,271,712 permits particles entrained in the steam path to pass through the "through opening" in its tip seal portion, this prior art design suffers from a number of significant shortcomings and drawbacks.
In particular, the introduction of one or more through-openings in the tip seal portion of the spillstrip ring about each turbine rotor causes a break in the tip seal. These breaks in the tip seal allow steam to flow therethrough which otherwise should pass over the blades of the rotor and impart torque to the turbine shaft. In addition, as particles entrained in the steam flow are directed outwardly against the outer diaphragm walls due to centrifugal forces acting thereupon, these particles are less likely to move radially inwardly where the through-openings in the tip seal are formed. These particles tend not to pass quickly through the through-opening(s) and downstream along the turbine where the particles can be effectively removed. Thus, with the spillstrip design proposed in U.S. Pat. No. 5,271,712, the resident time of particles at any particular stage of the turbine will be substantially higher than desired. Consequently, these particles are permitted to erosively damage the rotor blades during their extended residency between the rotor blades and diaphragm nozzles at each turbine stage.
Thus, there is great need in the art for a spillstrip design that can be used to create an improved tip seal in an axial flow elastic fluid turbine, while effectively reducing turbine part wear along the various stages thereof.